Preparation of photographic paper having good curl characteristics



I Patented Nov. 1953 PREPARATION or rno'roCnArnrC PAPER HAVING GOOD CURL CHARACTERISTICS Glen G. Gray, Rochester, N. Y., assignmto Eastman Kodak Company, Rochester, N. Y., a cor poratlon o1 New Jersey No Drawing. Application April 24, 1951,

Serial No. 222,746 I 7Claims. ((1117-34) Y This invention relates to a method of making photographic paper to minimize the curling characteristics thereof, which involves wetting the paper base before or at the time of the application of the emulsion coating thereon, followed by drying of the photographic paper.-

The preparation of photographic paper has been known for years, as illustrated by U. S. Patents Nos. 370,110 and 370,111 of Walkerand Eastman. Those patents describe a method of coating paper base with av photographic emulsion, followed by drying of the paper by movin it along at a slow speed and ordinary temperature. With the slow rate of drying used in the manufacture of photographic paper in those days, the curling characteristics of the paper thus prepared were not too unfavorable, particularly where the paper was stored in a place,

where there was not a great deal of variation in the relative humidity of the air in the place of storage. In recent years, however, the tendency has been to speed up the rate of drying of the emulsion-coated paper, which has involved the use of elevated temperatures in those drying operations.

The cellulose fiber in the paper base of photographic paper and the gelatin in the emulsion layer thereof are both hygroscopic. They expand as they adsorb water vapor at high-humidity, and contract as they desorb water vapor at low humidity. The rates of expansion and contraction of the two materials are not necessarily the same under a given set of conditions, so that the stresses and strains set up in photographic paper often result in curl. This is especially true in the case of light-weight paper stocks, suchas those having a weight of 8-15 pounds per thousand square feet. When such paper is coated with emulsion, often the stress in theemulsion layer is sufficiently great to result in face curl, or in other words, the curling of the paper is towards the emulsioncoated side.

When wet photographic emulsion is applied to dry paper, some of the'moisture from the emulsion penetrates the paper, resulting in expansion of the paper base, while loss of water by the emulsion causes its contraction. Thus,

the expansion of the paper and the contraction of the emulsion at that time sets up a balance of forces, somewhat diflerent from that which place.

stresses and strains resulting in emulsion-coated paper after the drying of that paper has taken Another object of my invention is to provide a method for manufacturing photographic paper, which paper will withstand widevariations in the relative humidity of the air with which it is in contact without any ap-- preciable curl. A further object of my invention is to provide a method of preparing photo-- graphic paper in which opposite forces are notset up by the paper base and the emulsion coating before and at the time of drying. Other objects of myinvention W111 appear herein.

I I have found that the stresses and strains in photographic paper may be prevented by imparting a moisture content to the wire side of the paper base prior to or upon the application. of the emulsion coating to the other side of that.

base, followed by a drying operation in which.

obtains in the paper and the emulsion immediately prior to the adsorption of water by the.

paper. A

One object of my invention is to minimize the both the paper and the emulsion are dried si-- multaneously.

paper is kept at a minimum.

In preparing photographic paper in accordance with my invention, the procedure which is followed is no different as far as applying 'the emulsion coating to the paper is concerned emulsion is applied to the paper it is preferred that the paper bechilled so as to set the emulsion, whereupon the paper may then be dried by the method described in the patents referred to, or preferably, by conducting the paper' through a chamber where it is subjected to acurrent of warm, dry air passing over the sur-.

face of the emulsion, all being carried out under darkened conditions. My invention involves the application of moisture to the paper base in I have found that the paperwhich results from-the practice of my invention. may 'be stored under widely variable relative. humidity conditions without exhibiting excessive: curl, as has been encountered previously with. photographic papers. 1 My invention is especially adaptable to photographic papers in which a light-weight'paper base is used, such as a paper base having a weight of ,8-15 pounds per thous and square feet. By means of my invention, the: influence of the emulsion-coated side under ex tremely dry conditions does not over-balance: the flattening tendency of the paper base underthose conditions so, that curling of the coated.

this procedure prior to the application of the emulsion coating thereto. This application of moisture to the paper may be, for instance, by means of an ordinary applicator roll, over which the paper sheet is conducted, or by subjecting the paper to a steaming operation. The moisture which is applied to the paper is applied to the wire side of the paper which is other than the side upon which the emulsion coating is applied. The paper which is employed for preparing photographic paper ordinarily has a moisture content of approximately percent. The use of a water applicator roll supplying moisture to the paper ordinarily brings the moisture content of the paper up to '7 percent or more, the additional moisture being concentrated on the side of the paper opposite that upon which the emulsion coating is applied.

Instead of using a water applicator roll, the paper may be steamed by directing a steam jet to the side of the paper opposite that upon which the emulsion coating is to be applied. In the coating operation for the manufacture of photographic paper, in accordance with my invention, either the water applicator roll or the steam jet may be conveniently provided as the first step in the operation immediately prior to the emulsion coating step, so that my invention is readily adaptable to a continuous method for the manufacture of photographic paper.

For example, a photographic paper was prepared by passing paper base first over a water applicator roll, imparting a water content to the paper of 7 percent, the added moisture being concentrated on one side of the paper. The opposite side of the sheet was then coated with a gelatin-silver halide photographic emulsion. The paper was subjected to treatment with chilled air, whereby the emulsion layer was set whereupon the paper was run through a chamber having a current of warm, dry air, the re sulting paper having good curling characteristics, as was determined by tests thereof. The curling characteristics of photographic paper were determined by means of discs 10 cm. in diameter cut from photographic paper sheets that had been conditioned at 18 percent relative humidity. To obtain the curl value of the paper under a given condition, the chord of the are formed by the disc when under that condition is measured and this value is subtracted from 10. For instance, a flat sheet would have a value of 0, while a paper which curled until the two portions met would have a value of 10. Where the disc curls more than once, the overlap is measured and added to 10. For instance, a disc curling 1 times would have a disc curl value of 15. If the disc curled twice, the curl value would be taken as 20. In testing the paper for curl after measurement of the discs in air having 18 percent relative humidity, they were placed in air having a '70 percent relative humidity, and after equilibrium was reached, measurements were again taken. The discs were then returned to air having 18 percent relative humidity and were measured again. The curl of the disc was also measured by placing such discs which had been conditioned at 18 percent relative humidity in a developer, and measuring the maximum curl occurring before penetration of the paper by the developer relaxed the stresses in the paper and the emulsion. This latter curl value was entitled processing curl. 4

The following examples illustrate the improvement which is obtained by my invention over paper which has been regularly coated and dried:

Example 1.-Photographic paper was prepared by the ordinary method of applying a photographic emulsion coating thereto and drying. Papers were also prepared using the same weight paper base. but in one case steaming the paper lightly on the wire side prior to the application of a gelatin-silver halide photographic emulsion layer to the opposite side thereof, and in another case, by moistening the paper on the wire side with a water applicator roll and then applying a gelatin-silver halide photographic emulsion layer on the opposite side of the paper. The regularly coated paper was designated A," that which was steamed lightly was denoted "B." and the paper which was moistened with a water applicator roll was designated "0." The results obtained were as follows:

Processing Curl won It will be noted that the application of moisture to the paper before applying the emulsion thereto gives significantly improved curl characteristics over paper which has been prepared without any such moistening operation in all the various conditions where the curl characteristics were measured.

Example 2.Photographic emulsion was applied to a paper base having a weight of 12 pounds per square foot. In one case, the emulsion was applied directly to the paper without any pre- VlOllS moistening thereof, chilled and dried. In a second case, the paper was wetted with a water applicator roll at substantially the same time as the application of the emulsion coat thereon. These samples were designated as A and B respectively. The curl characteristics This example illustrates the fact that the wetting of the side of the paper opposite the emuls on layer prior to the drying operation appreciably improves the curl characteristics of the paper, over the curling characteristics of paper prepared without any moistening operation occurring. The advantage in the hereindescribed method of preventing curl in paper is that such a method is readily adapted to any of the ordinary methods of coating photographic paper but with the addition that at, or preferably im n ediately prior to the application of the ernulsion coating to the paper, moisture is applied tothe paper on the opposite side to that on which the emulsion coat is applied, so as to increase the moisture content 0 (based on the paper) I f the paper 2-5 percent I claim:

1. A method of preparing photo a hi whereby the curling characteristic s t l ier o f s minimized which comprises applying to the paper a photographic emulsion coating and, on the opposite side of that paper, no later than the application of the emulsion coating. moisture in a sufiicient amount to increase the moisture content or the paper 2--5% based on the weight of the paper, followed by a drying operation in which the moisture content of both the paper and the emulsion is reduced.

2. In a method of preparing photographic paper in which a photographic emulsion. is applied to one side of the paper, the step which comprises applying water to the opposite side of the paper immediately prior to the application of the emulsion in sufiicient amount to increase the moisture content of the paper 2-5% based on the weight of the 139.1381.

3. A method of preparing photographic paper having good curl characteristics which comprises applying a photographic emulsion layer to one side of the paper and on the other side no later than the time of application of the photographic emulsion, sufiicient moisture to increase the moisture content of the paper 2-5% based on the weight of the paper by means of a water applicator roll followed by a drying'cperation in which the moisture content of ooth the paper I and the emulsion is reduced.

4. A method of preparing photographic paper having good curl characteristics which cornprises applying a photographic emulsion layer to one side of the paper and on the other side no later than the time of application of the photographic emulsion. sufficient moisture to increase the moisture content of the paper 2-570 based on the weight of the paper by means of a steam jet followed by a drying operation in which the moisture content of both the paper and the emulsion. is reduced,

5.. In a process of preparing photographic paper the step which comprises applying moisture to one side of the paper in a sufiicient amount to increase the moisture content of the paper 2 based on the weight of the paper immediately prior to the application of the photographic emulsion layer to the opposite side of the paper.

6. In a process of making photographic paper the step which comprises applying to one side of the paper sufiicient moisture to increase the moisture content of. the paper 2-5% based on the weight of the paper, at substantially the same point as the photographic emulsion layer is being applied to the opposite side of the paper.

GLEN G. GRAY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,229,741 Hinz Jan. 28, 1941 2,314,797 Morris et al. Mar. 23, 1943 2,425,653 Stewart Aug. 12, 1947 

1. A METHOD OF PREPARING PHOTOGRAPHIC PAPER WHEREBY THE CURLING CHARACTERISTICS THEREOF ARE MINIMIZED WHICH COMPRISES APPLYING TO THE PAPER A PHOTOGRAPHIC EMULSION COATING AND, ON THE OPPOSITE SIDE OF THAT PAPER, NO LATER THAN THE APPLICATION OF THE EMULSION COATING, MOISTURE IN A SUFFICIENT AMOUNT TO INCREASE THE MOISTURE CONTENT OF THE PAPER 2-5% BASED ON THE WEIGHT OF THE PAPER, FOLLOWED BY A DRYING OPERATION IN WHICH THE MOISTURE CONTENT OF BOTH THE PAPER AND THE EMULSION IS REDUCED. 